A silicon chip, or integrated circuit (IC), is the core element of an electronic device and usually comes in packaged form. With the development of manufacturing technology and requirement of compact design for end products, various packaging methods were invented to meet the demand. At most of the time, silicon chips are sealed within a protection material such as epoxy resin. There are certain cases, particularly when the silicon chip is a sensor device such as a fingerprint sensor chip, that the silicon chip needs to be mount on a substrate and has the surface exposed. Meanwhile, for a fingerprint reader device, the thickness of the packaged sensor must be as low as possible. Hence, the technique of bonding the silicon chip to the substrate plays a very important role. The bonding needs to ensure good circuit connectivity formed therebetween, and to be rigid enough to sustain the fingerprint reader sensor with shear force exerted by a finger.
Conventional wire bonding is applicable to the above requirement. Please refer to FIG. 1. A PCB 1 has an opening 2. A chip 3 in form of a die would like to be mounted on the PCB 1, over the opening 2. There are many attachment pads 4 on one surface of the chip 3. Some connectors 5 are arranged on the PCB 1. By wire bonding, gold wires 6 are formed to link the related attachment pads 4 and the connectors 5. In order to fix the PCB 1 and the chip 3, a layer of glue (not shown) may be applied on the interface between the PCB 1 and the chip 3. For some electronic devices, thickness is much concerned. Wire bonding for the PCB 1 and the chip 3 is not proper for them. This is because the height H of the gold wires 6 over the PCB 1 will occupy more space above the PCB 1 and make the thickness of the electronic device thicker.
Flip chip technology is another commonly used means to implement the above requirement. Take the same elements used in FIG. 1 for illustration. Please see FIG. 2. It should be noticed that near the end of the manufacturing process, the attachment pads 4 of the chip 3 are metalized to make them more receptive to solders. This typically consists of several treatments. Small dots of solder balls 7 are then deposited on each metalized pad 4. The chips 3 are then cut out of the wafer as normal. To attach the flipped chips 3 into the PCB 1, the chip 3 is inverted to bring the solder balls 7 down onto the connectors 5 on the underlying PCB 1. The solder balls 7 are then re-melted to produce an electrical connection, typically using a thermosonic bonding or alternatively a reflow solder process. This also leaves a small space between the chip's circuitry and the underlying mounting.
One challenge of the flip chip technology is heat dissipation for thermal stress in the chip 3. An adhesive (not shown) may be used as a heat bridge to ensure the solder balls 7 are not stressed due to differential heating of the chip 3 and the PCB 1. The adhesive distributes the thermal expansion mismatch between the chip 3 and the PCB 1, preventing stress concentration in the solder balls 7 which would lead to premature failure. When fan-outs of logic gates of a chip increases and associated pads for soldering become more and tiny, efficiency of heating dissipation of such adhesive for the solder balls get worse. Meanwhile, if the chip 3 is a fingerprint reader sensor chip, the binding ability between the chip and the PCB may not sufficient to resist the force exerted by a finger.
Therefore, an improved PCBA structure with a chip, especially an image sensor chip, mounted on a PCB over an opening is still desired. More particularly, the binding ability between the chip and the PCB should resist external force exerted onto the chip.